Patent GB-1041917-A (GB)
Title: Control apparatus for a television receiver
IPC: H04N9/455
Filed: 1964-06-30
Abstract: 1,041,917. Television. HAZELTINE CORPORATION. June 30, 1964 [Oct. 31, 1963], No. 26985/64. Heading H4F. The invention relates to control apparatus for a colour television receiver which separates deflection synchronizing pulses and/or colour synchronizing (burst) signals from a received composite colour signal and derives an automatic-gain-control (AGC) bias from the back porch level which thus eliminates black level shift which occurs in television receivers employing D.C. restoration since the back porch level differs from the black level by only a small fixed amount. As shown, the control apparatus 10 in a standard colour television receiver comprises means to derive a signal representative of variations in the amplitude of the horizontal back porch blanking level from the back porch level and to separate the colour burst. The colour burst and the horizontal back porch blanking level at least are supplied through resistance 11 to a control electrode of pentode 12 which operates as a keyed rectifier and when anode current is drawn by pentode 12 the colour burst present at the control grid is amplified and appears as a voltage across a tuned network 13 for separating the colour synchronizing signals. The output signal developed across network 13 is coupled through a capacitance 14 to a colour signal decoder circuit 15. Pentode 12 is keyed into conduction only during the back porch reference level by the application to the anode and screen electrode of positive going voltage pulses, i.e. horizontal flyback pulses A, Fig. 2b (not shown), which are obtained from the synchronizing signal separator and differentially delayed, the deflection signal generator circuits 37 being coupled through capacitor 36, delay circuit 16, 17, 18 and primary winding of transformer 13 to the anode of pentode 12, waveform B, and to the screen of the pentode through additional delay network 20, 21, waveform C. As anode current is drawn only when both the anode and screen electrodes have a potential which is positive with respect to the cathode potential pentode 12 is only conducted during the back porch interval. The amplified colour burst is included in waveform B, Fig. 2b (not shown), and represents a portion of the output signal developed at the anode. The pulse of anode current developed at the pentode 12 is used to derive, in a conventional AGC distribution network 24 which integrates the current pulse, an AGC bias voltage for the signal translator 25. Control apparatus 10 may include a cathode biasing network 27, 28, 29 for providing a fixed level of D.C. bias voltage at the cathode of pentode 12. In a further embodiment, Fig. 3 (not shown), control apparatus 10<SP>1</SP> in addition to deriving a signal used as an AGC bias and separating the colour burst from the supplied composite colour signal separates deflection synchronizing pulses as well by means of aperture means 30, which apertures on deflection synchronizing pulses and noise pulses. Undelayed horizontal flyback pulses are coupled to the anode of pentode 12 from the transformer winding 19 and only during the occurrence of these flyback pulses can the pentode draw anode current. Waveform A<SP>1</SP> represents anode voltage in Fig. 4b (not shown). The D.C. bias applied to the electrodes of pentode 12 is so adjusted that the variations of screen current are negligible except during the occurrence of deflection synchronizing pulses or impulse noise at the control grid and during these time intervals the screen current increases and voltage pulses constituting amplified, inverted deflection synchronizing pulses or impulse noise spikes are developed across resistance 32. These pulses are coupled through capacitance 35 to the deflection signal generator circuits 37<SP>1</SP>. The instantaneous variations of the screen voltage are coupled to the suppressor electrode through capacitance 34 and during the time occurrence of deflection synchronizing pulses oe noise spikes the voltage variations at the suppressor, Fig. 4c (not shown), prevent thr pentode 12 from conducting anode current. The anode current, Fig. 4d (not shown), is drawn only during the back porch interval and is used to establish an AGC bias. The pentodes output includes when conducting colour burst and deflection synchronizing pulses and a signal whose average value provides an AGC bias which is derived from the back porch level. In a further embodiment, Fig. 5 (not shown), delayed flyback voltage pulses are applied to the anode of the pentode instead of using pulse aperture means 30 to prevent the pentode from conducting, and a biasing network 38 is provided to vary the threshold level at which the delayed AGC bias is applied to a carrier signal translator 25.